Heterogeneously Catalyzed Reaction of a Jet-Exhaust Species with an Ambient Coflowing Species

Abstract

An approximate simple semi-empirical model is developed to bound the extent of heterogeneously catalyzed chemical reaction occurring between two initially separated gaseous species in an axisymmelric compound jet, i.e., a jet with averaged nozzle-exit speed greater than that of a uniform enveloping coflow. The dynamic properties of the compound jet are adopted from measurements reported by Maezynski (1962). The chemical reaction between species B (initially present in the issuing jet only) and a species A (initially present in the coflow only) occurs only on the surface of particles (initially present in the issuing jet only). The particles participate purely as catalysts; an upper bound on the extent of reactant conversion is obtained by taking the (unknown) rate of surface catalysis to be mass-transfer-limited. The results furnish quantitative estimates for a chemical reacting shear flow without resort to experimentally discredited gradient-diffusion concepts to model the turbulent transport. An application, in the context of a solid-rocket motor with a conventional metallized grain (ammonium-perchlorale crystals and aluminum spheroids in a synthetic-rubber binder), associates species A with chlorine nitrate (C1ONO₂), species B with hydrogen chloride (HO), and the particles with alumina (AI₂O₃), known to be heterogeneously catalytic in other contexts. The alumina might locally and transiently serve to generate the photochemicalty sensitive, potentially ozone-destructive species chlorine (O₂) from the photochemically insensitive (“chlorine-reservoir”) species HC1 and CIONO₂-The analysis suggests that even the upper bound on the amount of chlorine produced by alumina-particle catalysts (on the multihour time scale of a discernible-jet-exhaust flow) is negligible, owing to the diluteness of the ambient chlorine nitrate. Any longer-term, cumulative ozone destruction, arising because broadly dispersed alumina submicron-sized particles may fall out of the stratosphere only on a time scale of years, requires treatment of the general stratospheric circulation, and is not addressed by the analysis.